Protection against infection and abortion induced by virulent challenge exposure after oral vaccination of cattle with Brucella abortus strain RB51

Objectives - To determine efficacy of orally administered Brucella abortus vaccine strain RB51 against virulent B abortus challenge exposure in cattle as a model for vaccination of wild ungulates. Animals - 20 mixed- breed beef cattle obtained from a brucellosis-free herd. Procedure - Sexually mature, Brucella-negative beef heifers were vaccinated by mixing \u3e 1010 viable RB51 organisms or diluent with their feed. Heifers were fed individually and consumed their entire ration. Each heifer received approximately 3 X 1010 colony-forming units (CFU). Six weeks after oral vaccination, heifers were pasture-bred to brucellosis-free bulls. At approximately 186 days\u27 gestation, heifers were challenge exposed conjunctively with 107 CFU of virulent B abortus strain 2308. Results - Vaccination with the rough variant of B abortus RB51 did not stimulate antibodies against the O-polysaccharide (OPS) of B abortus. After challenge exposure and parturition, strain 2308 was recovered from 80% of controls and only 20% of vaccinates. Only 30% of the vaccinates delivered dead, premature, or weak calves, whereas 70% of the controls had dead or weak calves. Conclusions - Cattle vaccinated orally with the rough variant of B abortus strain RB51 develop significant (P \u3c 0.05) protection against abortion and colonization and do not produce OPS-specific antibodies. Clinical Relevance - Results encourage further investigation into use of strain RB51 to vaccinate wild ungulates (elk and bison) orally